eLoops are pretty good. They are pricey though, around 25 each.
Noctuas are pretty good as well, but they are pricey as well at 25 a pop.
Someone here mentioned the Helix, which I heard a while back was a copy of the AP-15. I would give those a shot as they are the cheapest generally, but I cannot vouch for their performance.

Once you start going into the semi-noisy area there are many that can compete.
Heard great things about Corsairs line of fans, not the silent editions, the regular ones, just downvolt them to your liking.

The real alternative (i.e the same class of product) to GT is not generic fans being discussed, but other industrial fans such as Delta, Sanyo Denki (San Ace), ADDA. Ball bearing, bigger motor, huge MTBF...

The only problem they are not low noise optimized as they are made to work forever in mainframes, servers and telecom boxes where noise is not issue, but heat, dirt and high temperatures.

The real alternative (i.e the same class of product) to GT is not generic fans being discussed, but other industrial fans such as Delta, Sanyo Denki (San Ace), ADDA. Ball bearing, bigger motor, huge MTBF...

The only problem they are not low noise optimized as they are made to work forever in mainframes, servers and telecom boxes where noise is not issue, but heat, dirt and high temperatures.